1. Field of Invention
The present invention relates to sensor cells and to sensors which incorporate such sensor cells.
2. Description of Related Art
Chemical sensors incorporating arrays of sensor cells including semiconductor transistors are known. Such sensors have typically used a silicon wafer as the substrate material. However, silicon is a relatively expensive material. Furthermore for certain types of sensors, such as biosensors, disposability of the sensor after use is an especially important issue as the biosensor can only be used once before disposal. When silicon is used as the substrate material, disposing of the used biosensors becomes more problematical.
Additionally, the difficulties associated with fabricating transistor arrays on silicon substrates are known to increase significantly with increase in the size of the array. Hence, with silicon substrates the tendency is for a high density of devices for any given size of array. For biosensors, this high packing density can be problematical because for many applications the active parts of the microelectronic chip incorporating the array must operate in a wet environment.
Many forms of chemical sensors, such as biosensors, have been proposed. One type of multi-biosensor comprises a pH sensor in the form of an array of four Ion Sensitive Field Effect Transistors (ISFET""s) in Urination with four Metal Oxide Silicon Field Effect Transistors (MOSPET""s) acting as source follower circuits. However, in order to provide sufficient isolation between the ISFET""s, the proposed array was relatively bully in size. Furthermore, an IFSET is a form of transistor and considerable problems arise in isolating such devices from a solution being tested. To alleviate the problems of isolation, the ISFET""s and MOSFET""s have been proposed to be fabricated on a silicon layer in the form of a number of discrete sites supported on a sapphire substrate. Sapphire was used as the substrate material because of its excellent electrical isolation properties. A protectional membrane was then formed over the gate surfaces of the ISFET""s, followed by membranes respectively sensitive to the compounds to be tested. The individual sensors so produced functioned as pH sensors and could be used to detect urea, glucose and potassium. However, as mentioned above, the sensor array was of relatively large size, measuring approximately 2 mm in width and 6 mm in length for a four sensor array. Furthermore, sapphire substrates can only be used to fabricate arrays to a certain size and it is well known that the concerns rating to the fabrication of arrays using silicon increase significantly with increase of array size. Additionally, the silicon and, in particular, the sapphire substrate materials are relatively expensive and therefore chemical sensors of the above type are extremely costly to fabricate. This cost aspect is particularly burdensome when considering that many types of sensors can only be used once before disposal. Moreover, these materials are not readily disposable, giving rise to significant environmental concerns regarding disposal after use.
More recently, sub-micron CMOS technology has been proposed for use as a biosensor array for DNA analysis. This technology has enabled an array of up to about 1000 sensor cells to be fabricated on a substrate having a size in the order of a few millimeters square. However, as the CMOS devices are fabricated on a silicon substrate, the proposed array has a high packing density. To isolate the active CMOS devices from the wet operating environment, a specific integrated reaction test chamber is provided in the form of a cavity arranged between two superimposed and hermetically sealed primed circuits. The DNA material to be analysed is separated into its two strands by heating and, using a biochemical process, the stands are labelled with a fluorescent molecule. An analyte containing the DNA strands is then placed in contact with the chip. If a DNA strand has a sequence matching that of a target arranged on an electrode of the sensor, hybridisation occurs which results in a physical localisation of the DNA sample onto the appropriate electrode of the chip. The chip is then rinsed and the sensor is read with a CCD camera. As the DNA strands have been labelled with a fluorescent molecule, relative brightness on the electrodes of the device indicates where bonding has occurred. Key issues in the applicability of such devices are recognised as materials compatibility, manufacturing and packaging in order to reliably deliver a wet-chip concept and these can be compromised by the requirement to achieve a high packaging density on the silicon substrate material. Also, as will be apparent from the above description, such biosensors are relatively expensive to manufacture.
Thin film transistors (TFT""s) are relatively inexpensive to manufacture as relatively cheap non-silicon substrates such as soda glass or plastic can be used. The use of a plastics substrate can provide additional benefits as it is a relatively disposable material. Furthermore, TFT""s can be readily fabricated as large area arrays and such technology has already found widespread application in industry, such as for example, in the manufacture of active matrix liquid crystal display devices. The manufacturing processes are therefore well proven and a high yield of operable devices can reliably be obtained at relatively low costs, especially in comparison to silicon substrate devices. These advantages are further enhanced when considering that arrays larger than those available from silicon substrates can also be reliably fabricated. The use of silicon wafer substrates for such large area arrays is considered to be extremely problematical as it becomes increasingly difficult and expensive to fabricate the arrays in view of the substrate material itself and the semiconductor fabrication techniques which must necessarily be employed.
There are also drawbacks associated with the performance of such devices when used to sense certain substances. MOSFET""s typically comprise a relatively thin layer of silicon dioxide (SiO2) supported on a doped silicon substrate. The SiO2 layer has inherent capacitance which is inversely proportional to the thickness of the layer. If the SiO2 layer is fabricated to a typical thickness of about 100 nm, there is significant loss of capacitive signal from the device which is due to the inherent capacitance of the SiO2 layer. If the SiO2 layer is fabricated as a very thin layer to improve signal output, the devices become very unstable in use. These design conflicts can be alleviated if the sensing electrode is made very small. However, the sensing electrode must be fabricated to a practical size as it is used to receive the substance being identified. The MOSFET gate area must therefore be mad relatively large but this gives rise to the basic fabrication concern regarding the use of silicon transistors for chemical sensors in that the provision of relatively large gate areas significantly reduces the packing density of the transistors which can be accommodated on the finite size silicon substrates, which in turn reduces the number of sensor cells that can be accommodated in the sensor array.
For chemical or biosensors in particular, the ability of TFT""s to be readily fabricated as large area arrays at relatively low cost presents significant advantages in comparison to the conventionally used silicon devices as the need to achieve a very high packing density is not a dominant factor in device design. Hence, the area associated with each sensor cell of an array which receives the sample to be identified can, if necessary, be displaced from the active semiconductor components, alleviating the isolation concerns which exist with the current silicon substrate devices. Furthermore, the sensing areas for receiving a sample to be identified, which may be in the form of electrodes for a DNA sensor, can be made relatively large in size, enlarging the sensing area and enhancing device performance. Additionally, the use of enlarged sensing areas can provide a further benefit in that the packing density of the TFT""s can be reduced from that found in many current applications where these devices are used providing increased yields of fully functional devices from the existing fabrication processes.
TFT""s are known to exhibit lower mobility than silicon substrate transistors and, when fabricated as a large array of transistor devices, which would be of particular benefit for a biosensor, TFT""s can exhibit variations in transfer characteristic between the transistors in the array. These variations can become more pronounced as the array size is increased and for DNA biosensors in particular, where typically a very large number of samples need to be analysed to identify a sample, a large area array is of very significant benefit in reducing the time required to analyse samples.
Hence, it has been further realised with a preferred form of the present invention that, if the capacitance arising between an electrode and a sample to be identified is used as a measurement technique, the potential drawbacks associated with the variability in TFT performance can be overcome, enabling such devices to be readily used as the active devices for a chemical sensor in the form of a large array of sensor cells.
The use of TFT""s for chemical sensors not only provides the cost benefit over the use of silicon substrate devices but also provides the ability to fabricate large area arrays with enhanced sensing areas. Furthermore, there is also the significant additional benefit of improved disposability, which is particularly important for biosensor or chemical sensor devices because, as stated above, such devices can usually be used once only before disposal.
It is therefore an object of the present invention to provide an improved sensor cell utilising thin film transistors. Furthermore, it is also an object of the present invention in which detection of the capacitance on an electrode arising from the electrode receiving a sample for identification is used as the measurement technique and this capacitance is used to control the operation of the thin film transistors.
According to a first aspect of the present invention, there is provided a sensor cell comprising a thin film transistor and receiving means coupled to a gate electrode of the thin film transistor for receiving a sample for identification.
In a preferred arrangement, the sensor cell comprises a reference capacitor and the sample electrode and the reference capacitor are arranged as a capacitance divider circuit coupled to a gate electrode of the thin film transistor for controlling the amplitude of a voltage pulse provided to the gate electrode in dependence upon the value of capacitance arising at the sample electrode.
In an advantageous structure for the sensor cell, the reference capacitor comprises the gate electrode and a buried region underlying the gate electrode and separated therefrom by an insulator layer.
Preferably, the receiving means comprises a sample electrode, the arrangement being such that operation of the thin film transistor is controlled in dependence upon a value of capacitance arising at the sample electrode in response to receipt by the sample electrode of the sample for identification.
In an alternative arrangement, the sensor cell comprises a switching transistor for switching between a conducting condition and a non-conducting condition and wherein the thin film transistor includes a gate electrode, the arrangement being such that a voltage provided to the gate electrode with the switching transistor in the conducting condition reduces in magnitude in dependence upon the value of the capacitance arising at the sample electrode when the switching transistor is switched to the non-conducting condition.
Preferably, in this first aspect of the present invention, the sensor cell comprises a select line for providing a select pulse for switching the switching transistor between the conducting and non-conducting conditions and a write line for providing the voltage to the gate electrode of the thin film transistor, a read line for providing a read voltage to the thin film transistor, the arrangement being such that a write cycle is enabled by providing the select pulse to the switching transistor, thereby to switch the switching transistor to a conducting condition to enable the voltage to be provided to the control gate of the thin film transistor, and wherein a read cycle is enabled by terminating the select pulse thereby to switch the switching transistor to the non-conducting condition, whereby the voltage at the gate electrode of the thin film transistor changes in magnitude, thereby to switch the thin film transistor to a non-conducting condition for terminating the provision of an output signal from the thin film transistor, the time taken between termination of the select pulse and switching of the thin film transistor to the non-conducting condition being dependent upon the value of capacitance at the sample electrode.
Advantageously, the sensor cell may include a threshold voltage compensation circuit including a constant current source for providing a preset level of current through the film transistor and switching means for selectively coupling the constant current source to the thin film transistor.
Most advantageously, the sensor cell comprises an additional transistor coupled to the thin film transistor, the arrangement being such that when the voltage pulse is provided to the gate electrode of the thin film transistor and tie constant current source is decoupled from the thin film transistor, the magnitude of the output current from the thin film transistor will change from a first level determined by the constant current source to a second level in dependence upon the value of capacitance arising at the sample electrode
Advantageously, the receiving means is arranged in a position offset from the thin film transistor, the arrangement being such that the sample is received by the receiving means in a position which does not overlie the gate region of the thin film transistor.
Preferably, the sensor cell is fabricated on a plastics substrate.
According to a second aspect of the present invention there is provided a sensor comprising an array of rows and columns of sensor cells in accordance with the first aspect of the present invention.
According to a third aspect of the present invention there is provided a method for identifying a sample comprising providing a sensor cell including a thin film transistor and a sample electrode for receiving the sample and controlling the operation of the thin film transistor in dependence upon a value of capacitance arising at the sample electrode from receipt by the sample electrode of the sample.
Preferably, the method comprises providing a reference capacitor and arranging the reference capacitor and the sample electrode as a capacitance divider circuit coupled to the gate electrode of the thin film transistor and controlling the amplitude of a voltage pulse afforded to the gate electrode in dependence upon the value of capacitance arising at the sample electrode.
Advantageously, in this second aspect of the present invention the method comprises coupling the sample electrode with a switching transistor for switching between a conducting condition and a non-conducting condition, providing a voltage to a gate electrode of the thin film transistor with the switching transistor in the conducting condition, and coupling the sample electrode to the switching transistor whereby when the switching transistor is switch to the nonconducting condition the voltage provided to the gate electrode of tie thin film transistor changes in magnitude in dependence upon the vale of the capacitance arising at the sample electrode.
Preferably, the switching transistor is switched between the nonconducting and conducing conditions by providing a select pulse from a select line to the switching transistor and a write line is provided for providing the voltage to the gate electrode of the thin film transistors a read line for providing a read voltage to the thin film transistor, enabling a write cycle by providing the select pulse to the switching transistor, thereby to switch the switching transistor to a conducting condition to provide the voltage to the control gate of the tin film transistor, and enabling a read cycle by terminating the select pulse thereby to switch the switching transistor to the non-conducting condition, whereby the voltage at the gate electrode of the thin film transistor changes in magnitude, thereby to switch the thin film transistor to a nonconducting condition and terminate an output signal from the thin film transistor, the time taken between the termination of the select pulse and switching of the thin film transistor to the nonconducting condition beg dependent upon the value of capacitance at the sample electrode.
Most preferably, the method comprises providing the thin film transistor on a plastics substrate.
Advantageously, the method also comprises coupling an additional transistor to the thin film transistor, providing the voltage pulse to the gate electrode of the thin film transistor and decoupling the constant current source from the thin film transistor thereby to change the magnitude of the output current from the thin film transistor from a first level determined by the constant current source to a second level in dependence upon the value of capacitance arising at the sample electrode.
According to a fourth aspect of the present invention, there is provided a biosensor comprising a sensor cell according to the first aspect of the present invention or a sensor according to the second aspect of the present invention.
According to a fifth aspect of the present invention, there is provided fingerprint recognition apparatus comprising a sensor cell according to the first aspect of the present invention or a sensor according to the second aspect of the present invention.
According to a sixth aspect of the present invention, there is provided a method of operating a biosensor or fingerprint recognition apparatus according to the third aspect of the present invention.